68 LIGHTING AND FOCUSING \_CH. II 



liable to get on the cover-glass. If the objective is adjustable, fol- 

 low the directions given in § 114. 



When one is through using a water immersion objective, 

 remove it from the microscope and with some lens paper wipe alt 

 the water from the front lens. Unless this is done dust collects and 

 sooner or later the front lens will be clouded. It is better to use 

 distilled water to avoid the gritty substances that are liable to be 

 present in natural waters, as these gritty particles might scratch the 

 front lens. 



HOMOGENEOUS IMMERSION OBJECTIVES : EXPERIMENTS 



§ 116. As stated above, these are objectives in which a liquid 

 of the same refractive index as the front lens of the objective is 

 placed between the front lens and the cover-glass. 



§ 117. Tester for Homogeneous Liquid. — In order that 

 full advantage be derived from the homogeneous immersion prin- 

 ciple, the liquid employed must be truly homogeneous. To be sure 

 that such is the case, one may use a tester like that constructed by 

 the Gundlach Optical Co. , then if the liquid is too dense it may be 

 properly diluted and vice versa. For the cedar oil immersion liquid, 

 the density may be diminished by the addition of pure cedar wood 

 oil. The density may be increased, by allowing it to thicken by- 

 evaporation. (See H. L. Smith, Proc. Amer. Soc. Micr. , 1885, p. 

 83, and Ch. X.) 



§118. Refraction Images. — Put a 2 mm. ( T a T in.) homo- 

 geneous immersion objective in position, 'employ an illuminator. 

 Use some histological specimen like a muscular fiber as object, 

 make the diaphragm opening about 9 mm. in diameter, add a drop- 

 of the homogeneous immersion liquid and focus as directed in § 83. 

 The object will be clearly seen in all details by the unequal refrac- 

 tion of the light traversing it. The difference in color between it 

 and the surrounding medium will also increase the sharpness of the 

 outline. If an air bubble preparation (§ 88) were used, one would 

 get pure, refraction images. 



§ 119. Color Images. — Use some stained bacteria as Bacillus 

 tuberculosis for object. Put a drop of the immersion liquid on the 

 cover-glass or the front lens of the homogeneous objective. Re- 



